Astronomy originated as simple observation and recording of regular celestial movements for the purpose of time keeping and determining directional bearings. The ancient Greeks were among the first to study astronomy in detail. They described the use of stars for navigation at sea, and recorded the position of constellations during each season of the year for purposes of determining times for planting and harvesting. The Greek astronomer Aristarchus developed the theory that celestial motion could be explained by the fact that the Earth revolved on its axis once every 24 hours, and revolves around the sun along with the other planets. This theory was rejected at the time, only to be rediscovered nearly 2000 years later by Copernicus. These early students of astronomy compiled data that became the cornerstone for modern astronomical research.